1. The Field of the Invention
The present invention relates to drill heads and to drill heads configured to generate oscillating vibratory forces.
2. The Relevant Technology
Sonic head assemblies are often used to vibrate a drill string and the attached coring barrel and drill bit at high frequency to allow the drill bit and core barrel to penetrate through the formation as the drill bit rotates. Accordingly, some drilling systems include a drill head assembly that includes both an oscillator to provide the high frequency input and a motor driven gearbox to rotate the drill string. The sonic head includes pairs of eccentrically weighted rotors that are rotated to generate oscillating or vibratory forces. The eccentrically weighted rotors are coupled to a spindle. The spindle can in turn be coupled to a drill rod such that turning the eccentrically weighted rotors transmit a vibratory force from the spindle to the drill rod.
The force generated by the sonic head depends, at least in part, on the eccentric weight of the rotors, the eccentric radius of the eccentric weight of the rotors, and the rotational speed of the eccentric rotors. In most systems, the eccentric weight and eccentric radius of the rotors are fixed. Accordingly, in order to vary the vibratory forces generated by a given sonic head, the rotational speed of the eccentric rotors is varied. Each system has a natural harmonic frequency at which the vibratory forces resonate through the system resulting in extremely large forces. As the sonic head spins the rotors up to the desired rotational speed to apply a selected vibratory force, the system often passes through one or more of the harmonic frequencies. The forces generated at these harmonic frequencies are often large enough to damage the sonic head and other parts of the drilling system. The maximum force output of the oscillator can thus be dictated by the speed of rotation, which can be held below a speed corresponding to a harmonic frequency.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced.